Substructure Temporary Works

Temporary Works refer to any temporary structures, equipment or supports which are needed to facilitate safe construction and enable building the permanent structure.

It is usually the main contractors responsibility to assess site conditions and identify any requirements for temporary works.

This is often done with input from the specialist Groundworks Subcontractors and temporary works engineer.

Most sites will require a designated temporary works co-ordinator who will be responsible for ensuring compliance to HSE guidelines:

The law says any temporary structure must be designed, installed, and maintained to withstand any foreseeable loads which may be imposed on it and that it be only used for the purposes for which it was designed, installed, and maintained.

It’s worth identifying who this is for your project and asking them to brief you on some of the site-specific temporary works requirements.

When it comes to substructure, the following may need to be considered:

1. Battered excavations – where excavation is required to construct substructure (which is most of the time) it is important to consider how this will be done safely to reduce risk of the ground collapsing.

One option is to use battering - when the sides of the excavation are inclined at an angle away from the vertical axis.

Different ground types require different angles for them to be safe. This is known as the angle of repose i.e a stronger soil will be safer battered to a steeper angle.

2. Sheet Piling - long, interlocking structural sections made of steel that are driven into the ground to create a retaining wall.

If the requirement is to do a deep dig and battering is not an option, sheet piles may have to be installed.

Sometimes these can be pushed into the ground with an excavator. Alternatively, if the ground conditions or length of the sheet piles don’t allow this a specialist sheet piling rig may need to be used.

3. Formwork - also known as shuttering, refers to the temporary structure or mould used to shape and support freshly poured concrete until it sets and gains sufficient strength to support itself.

Commonly constructed using timber as image above with pins in the ground to hold it in place.

If the concrete is to be poured in the ground a twin wall polypropylene sheet system can be used to hold the concrete - this is referred to as Cordek on site after the brand that produces these.

4. Underpinning – supporting an adjacent building where the original foundations are no longer strong enough to support it.

If excavation is required close to the boundary there is a risk of undermining their foundations. Especially if the building is dated.

The solution may be to dig under the building and pour concrete to reinforce their foundations. However, if it was done all at once the building would likely collapse.

A hit and miss methodology is applied to dig out sections at a time and pour concrete in. A ‘key’ of some description is formed between pins, so they are joined.

In the drawing above all the rectangular concrete sections, knows as 'pins' numbered '1' would be excavated by hand and poured first. Once the concrete for these set and grout is installed to fill any gaps the number 2 pins would be dug.

5. Dewatering Systems - required in cases where excavations are at risk of water ingress. They pump water out to a designated point where it can be safely discharged, a license for the company who own the asset being discharged into may be required.

Definitions

Permanent structure - what is actually being built

Retaining wall – structures for supporting soils laterally designed to restrain soil to a slope that it would not naturally keep to

HSE - Health and Safety Executive. Britain's national regulator for workplace health and safety

Posts to Follow

We plan to go into more details on Substructure and add sections on:

·       Build Sequence

·       Health, Safety & Environment

·       Quality

·       Programme

·       Commercial

Please let us know if you found this post useful using the chat function. We'd love to hear from you - is there anything in particular you'd like for us to focus on next?